Thus R. Ishmael and Rabbi Akiba (Sanhedrin 4b) discuss the source of the requirement to insert four sections of the Shema in the phylacteries. Rabbi Akiba maintains that the word totafot itself implies four, for tot means “two” in Katpi [possibly a Coptic language], and fot means “two” in an ancient North-African language. Rashi (Deuteronomy 6:8) remarks that it indicates the four parashiyot in the tefillin as well as the four separate compartments [in the head phylactery] into which they are placed.
These and other questions are answered through exposition on various verses and Halacha leMoshe miSinai. [See Tractate Menachot 34b-38a and Rambam, Hilchot Tefillin, for further details.]
You have touched upon a very important concept without which we would be unable to understand or perform many of the mitzvot. We could point to a similar case regarding the etrog on Sukkot, as well as numerous other precepts. The details are all derived through exposition and Halacha leMoshe miSinai.
We might also add that when we – the person who reads from the Torah and the person called up to the reading, i.e., the oleh laTorah – see these seven letters written with coronets on the Torah scroll parchment, we are reminded that the Written Law is bound to the Oral Law. This constant reminder is such a great aid in improving our yir’at Shamayim (awe and fear of Heaven).